The present invention relates to a zoom lens and a video camera using the same. More specifically, the present invention relates to a miniature aspherical zoom lens with a high magnification (14 times or more) and a short optical full length, and a video camera using the same.
With widespread use of a digital video (DV) format, miniaturization and high image quality are important elements in commercial video cameras. Therefore, zoom lenses to be mounted thereon also are required to be small while achieving high image quality, having a short optical full length and a high zoom ratio.
For example, JP 9-281392 A discloses a zoom lens of high image quality with a zoom ratio of about 10 times.
The zoom lens disclosed by JP 9-281392 A realizes miniaturization and high image quality with a lens structure including only ten lenses; however, its zoom ratio is about 10 times.
In order to realize a higher zoom ratio while maintaining high image quality, it is required to use at least ten lenses or enlarge an optical full length, which makes it impossible to obtain a miniature zoom lens.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a zoom lens having a zoom ratio of 14 times or more with a smaller number of lenses, and a miniature video camera of high image quality using the zoom lens.
In some embodiments, the zoom lens of the present invention includes: a first lens group having positive refracting power and fixed with respect to an image plane; a second lens group having negative refracting power and varying power, wherein the power is varied by moving along an optical axis; a third lens group having positive refracting power and fixed with respect to the image plane; and a fourth lens group having positive refracting power and moving along an optical axis so as to keep the image plane at a predetermined position from a reference surface, the first, second, third and fourth lens groups being disposed from the object side to the image plane side in this order, wherein the second lens group includes at least one aspherical surface, the third lens group is composed of three lenses including two positive lenses and one negative lens and includes at least one aspherical surface, and the fourth lens group is composed of a positive lens including at least one aspherical surface.
In some embodiments, the third lens group is composed of three lenses: two positive lenses and one negative lens, whereby a miniature zoom lens is realized, in which spherical aberration is corrected satisfactorily from a wide-angle end to a standard position. Furthermore, by disposing at least one aspherical surface in each of the second, third and fourth lens groups that have a small lens diameter and adopting optimum aspherical surface shape and lens type, a miniature zoom lens with a zoom ratio of 14 times or more can be realized with a lens structure using a small number of lenses, in which an aberration is corrected satisfactorily.
In some embodiments, the third lens group is composed of three lenses including a positive lens,-a positive lens and a negative lens disposed from the object side in this order.
In some embodiments, the third lens group is composed of three lenses including a positive lens, and a cemented lens of a positive lens and a negative lens disposed from the object side in this order.
In some embodiments, the third lens group is composed of three lenses including a positive lens, a negative lens, and a positive lens, disposed from the object side in this order.
In some embodiments, the third lens group is composed of three lenses including a positive lens, and a cemented lens of a negative lens and a positive lens disposed from the object side in this order.
In some embodiments, the second lens group is composed of three lenses including a first negative lens, a second negative lens and a positive lens disposed from the object side in this order, a surface on the object side of the second negative lens is aspherical, and a local radius of curvature R10 in the vicinity of an optical axis and a local radius of curvature R11 in an outer peripheral portion satisfy the following conditional expression (1):
0.5 less than R11/R10 less than 0.95xe2x80x83xe2x80x83(1)
Using this example, a coma aberration at a wide-angle end and spherical aberration at a telephoto end are corrected satisfactorily.
In some embodiments, a focal length f3 of the third lens group and a focal length fw of an entire system at a wide-angle end satisfy the following conditional expression (2):
2.5 less than f3/fw less than 4.0xe2x80x83xe2x80x83(2)
Using this example, a miniature zoom lens is realized, which ensures a back-focus enabling a crystal filter, an IR cut filter, and the like to be inserted.
In some embodiments, a surface on the object side of a lens disposed closest to the object side in the third lens group is aspherical, and a local radius of curvature R20 in the vicinity of an optical axis and a local radius of curvature R21 in an outer peripheral portion satisfy the following conditional expression (3):
1.05 less than R21/R20 less than 3.5xe2x80x83xe2x80x83(3)
Using this example, a zoom lens is realized, in which a spherical aberration in an entire zoom region is corrected satisfactorily.
In some embodiments, an absolute value R30 of a smaller radius of curvature of a concave lens included in the third lens group, and a focal length f3 of the third lens group satisfy the following condition (4):
0.35 less than R30/f3 less than 0.8xe2x80x83xe2x80x83(4)
Using this example, a zoom lens is realized, in which a coma aberration is corrected satisfactorily in rays outside an off-axis principal ray.
In some embodiments, a focal length f4 of the fourth lens group and a focal length fw of an entire system at a wide-angle end satisfy the following conditional expression (5):
2.5 less than f4/fw less than 6.0xe2x80x83xe2x80x83(5)
Using this example, a miniature zoom lens is realized, which ensures a back-focus enabling a crystal filter, an IR cut filter, and the like to be inserted.
In some embodiments, a surface on the object side of a lens in the fourth lens group is aspherical, and a local radius of curvature R40 in the vicinity of an optical axis and a local radius of curvature R41 in an outer peripheral portion satisfy the following conditional expression (6):
1.05 less than R41/R40 less than 1.75xe2x80x83xe2x80x83(6)
Using this example, a zoom lens is realized, in which a coma aberration is corrected satisfactorily in rays inside an off-axis principal ray.
Furthermore, the video camera of the present invention is provided with the zoom lens of the present invention. According to this structure, a miniature video camera of high image quality with a high zoom ratio is realized.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.